Post-translational Modifications of SIBLING Proteins and Their Roles in Osteogenesis and Dentinogenesis

Abstract

ABSTRACT:The extracellular matrix (ECM) of bone and dentin contains several non-collagenous proteins. One category of non-collagenous protein is termed the SIBLING (Small Integrin-Binding LIgand, N-linked Glycoprotein) family, that includes osteopontin (OPN), bone sialoprotein (BSP), dentin matrix protein 1 (DMP1), dentin sialophosphoprotein (DSPP), and matrix extracellular phosphoglycoprotein (MEPE). These polyanionic SIBLING proteins are believed to play key biological roles in the mineralization of bone and dentin. Although the specific mechanisms involved in controlling bone and dentin formation are still unknown, it is clear that some functions of the SIBLING family members are dependent on the nature and extent of posttranslational modifications (PTMs), such as phosphorylation, glycosylation, and proteolytic processing, since these PTMs would have significant effects on their structure. OPN and BSP are present in the ECM of bone and dentin as full-length forms, whereas amino acid sequencing indicates that DMP1 and DSPP exist as proteolytically processed fragments that result from scission of X-Asp bonds. We hypothesized that the processing of DMP1 and DSPP is catalyzed by the PHEX enzyme, since this protein, an endopeptidase that is predominantly expressed in bone and tooth, has a strong preference for cleavage at the NH 2 -terminus of aspartyl residue. We envision that the proteolytic processing of DMP1 and DSPP may be an activation process that plays a significant, crucial role in osteogenesis and dentinogenesis, and that a failure in this processing would cause defective mineralization in bone and dentin, as observed in X-linked hypophosphatemic rickets.

Modulation of extracellular matrix protein phosphorylation alters mineralization in differentiating chick limb-bud mesenchymal cell micromass cultures

“…This suggests that the inhibitors were functioning as predicted, and also that extracting with 2M guanidine hydrochloride enriched the extracts with these phosphorylated proteins. Each of these proteins can be nucleators of and/or inhibitors of hydroxyapatite formation and crystal growth, depending on their concentrations, posttranslational modifications, and interactions with other molecules [9].…”

“…While the importance of the phosphate ion for appropriate mineral deposition is well recognized [7], it is only in recent years that attention has been paid to whether or not the extracellular matrix proteins were phosphorylated, as well as to their extent of phosphorylation and the enzymes involved in these posttranslational modifications [1][2][3][4][5][6][7][8][9]. The importance of phosphorylation and dephosphorylation has been suggested by the abnormal patterns of mineralization in mutant animals with defective phosphoprotein degradative enzymes [10][11][12], defective phosphate transport [13,14], or defective phosphorylation [15].…”

Proteolytic processing of osteopontin by PHEX and accumulation of osteopontin fragments in Hyp mouse bone, the murine model of X-linked hypophosphatemia

“…(16)(17)(18)(19) Another processing step likely to be important in modifying the function of bone and tooth matrix proteins is their proteolytic cleavage by enzymes. (20) Such processing can potentially have effects on mineralization by generating constituent bioactive fragments acting locally within the extracellular matrix (such as the ASARM peptide) (13,14,21) or acting both locally and systemically on mineralization as seen for dentin matrix protein 1 (DMP1) after proteolytic processing by bone morphogenetic protein 1 (BMP1). (22)(23)(24)(25) OPN is a highly phosphorylated protein belonging to a family of very acidic proteins found in bones and teeth (and in pathologically calcified soft tissues) that bind to mineral and are called the small, integrin-binding ligand, N-linked glycoproteins (SIBLINGs).…”

Mineralization and non-ideality: on nature’s foundry

“…Secreted forms of kinases tweak the phosphorylation of extracellular proteins and thereby guide biomineralization processes (Tagliabracci et al 2012). Matrix metalloproteases are important regulators of biomineralization, for instance by suppressing the occlusion of biomolecules in biominerals and regulating the morphology of mineral particles (Prajapati et al 2016;Qin et al 2004). Thus, the mineralization 'activity' of biomolecules in terms of their spatial distribution and also interactions with inorganic phases are modulated by regulatory checkpoints.…”